Selective tuning system



Dec. 15, 1936. R A H 2,063,965

' SELECTIVE TUNING SYSTEM I I Original Filed June 29, 1927 Illllll l I l r I I INVENTO R PM J. 142/5 \S HIS ATTORNEY Patented Dec. 15, 1936 iiii'ilfifi STATES PATENT @FFEQE Application June 29, 1927, Serial No. 202,390

Renewed December 31, 1934:

Claims.

This invention relates to radio receiving systems, and especially to a scheme whereby such systems can be made highly selective.

In connection with radio communication, one

5 of the important problems relates to the separation of signals from various sources. This problem is at the present time most acute in connection with broadcast reception, where many stations are operating on frequencies that are so close together as to make it difficult to select one of them to the total exclusion of the others.

Such broadcast systems transmit modulated radio frequency carrier waves. When they are picked up by an antenna or equivalent pick-up circuit, the impulses are amplified and detected. It is evident that when two or more stations are within the operative range, they can all affect the receiving system. It has been proposed in the past to select or at least accentuate the desired impulses by impressing all the received signals on a cascade amplifier system operating on the signals prior to detection; the cascade system incorporating a succession of circuits in the path of the impulses, and each tuned to the desired frequency. The impulses in passing through one tuned circuit, are usually transmitted to an am- 7 plifier, such as an audion and the amplified impulses are impressed upon another tuned circuit.

This procedure is intended to be continued until 30 there is a sufficient accenuation of the desired signals over the undesired signals.

However, it has been found that such an old system, even if not entirely inoperative, is in many respects impractical. In the first place,

35 even a few stages of such radio frequency amplification would cause very serious instability; parasitic oscillations would arise that are hard to overcome.

Furthermore, in such a system, the process of 40 selection does not involve the discarding of the undesired impulses; it merely provides that these undesired impulses are not amplified to as great an extent as the desired impulses. For this reason, numerous stages must be provided, with all 45 its attendant objections, to secure the desired degree of selectivity.

Perhaps the most objectionable feature of this system of tuned radio frequency resides in the fact that each of the tuned circuits to be at all 50 efiective, must be sharply tuned. This requires that the circuits be tuned separately, one by one; a time-consuming and laborious process. Of course, a common actuator could be used for all the tuning devices,but experience shows that it is 55 difficult to maintain all of the circuits simultaneously in exact resonance over the range of operation.

It is one of the objects of my invention to obviate these disadvantages, and to provide a simple scheme for obtaining a high degree of selectivity. 5

It is another object of my invention to produce selectivity, not so much by accentuation of the desired impulses, but rather by cancelling out the undesired impulses.

It is still another object of my invention to pro- 10 vide a system that is highly sensitive and that can be operated by a single dial.

It is still another object of my invention to provide a selective receiving system that combines a high degree of selectivity and stability, without 15 the necessity of utilizing neutralization or stabilization.

My invention possesses many other advantages, and has other objects which may be made more easily apparent from a consideration of one 20 embodiment of my invention. For this purpose,

I have shown a form in the drawing accompanying and forming part of the present specification.

I shall now proceed to describe this form in detail, which illustrates the general principles of my invention; but it is to be understood that this detailed description is not to be taken in a limiting sense, since the scope of my invention is best defined by the appended claims.

Referring to the drawing: 7

The single figure is a schematic wiring diagram of a receiving system embodying my invention.

I show a pick-up system including in this instance the elevated conducto-rs or antenna l and a ground connection 2. I arrange the pick-up system however, in a novel manner, which makes it possible to cancel out substantially entirely, the interfering impulses received in the pick-up system. In the old forms of pick-up systems, they were left either untuned, or else they were tuned to form a series resonant circuit, in which the open capacity between the antenna and ground was included. In this way, all the energy received in the pick-up system passed through the entire circuit. With my arrangement, I provide a scheme whereby the desired impulses are confined to a definite portion only of the pick-up system.

This is accomplished by the aid of a parallel resonant circuit 3 adjacent the antenna l and located therein. This circuit may include an inductance 4 and a variable capacity 5 paralleling the inductance. The circuit 4-5 is tuned to the frequency of the desired impulses. If there is negligible resistance in this tuned circuit, practically all of the energy received in the pick-up system at the desired frequency will circulate in this circuit, and substantially none of it will be passed on to the coil 6 which is in series with the ground 2. It is to be noted particularly that the entire antenna circuit l 52 is not tuned to the frequency of the desired impulses; a portion only of it is so tuned for the purpose of providing a trap for these impulses.

The undesired impulses do, however, pass through coil 6 and to ground 2. This coil utilized as a means for providing a cancelling out efiect in the circuits associated with circuit 3, in a. manner that will be hereinafter described in detail.

The desired impulses produce relatively high potential dirferences across the coil 6 or condenser 5. These potential difierences can be directly impressed, if desired, on the input side of an amplifier l, shown in this instance as an electronic emission device, such as an audion. This amplifier is in the form of an evacuated vessel, enclosing an electron emitting electrode such as a heated filament 3, whence electrons fiow to a plate or anode 9 also enclosed in the vessel. t has been found that if the plate 9 is maintained positive with respect to filament S, then a space current is set up between these electrodes, which can fiow through an external path connecting the electrodes. This path includes filament 8 (which is heated by current from battery ii? or other source), connection H, at least a por ion of the source of positive potential i2, connection l3, choke coils i l and i5, capacity it and inductance H in parallel, and plate 9. The capacity it can represent either a separate condenser, or the distributed capacity of coil ll. For this purpose, a coil can be chosen having a fixed natural wave-length of about the average value of the impulses to be received; for broadcast reception, about 306 or 400 meters.

The impedance of the path for the electronic or space current between filament 8 and plate 9 can be very greatly varied by the aid of a third control electrode i8, such as a grid, which can affect the potential of a point in the space of the tube. When the potential difierence between the grid 58 and filament 8 is varied even slightly, there is a corresponding large variation in tube impedance, and it is this feature which makes it possible to ainplify'impulses. The grid l8 and filament 3 are connected respectively to opposite terminals of condenser 5, so as to cause these controlling potential variations between them. Since the operation of such amplifier tubes as l is now well understood, further detailing thereof is unessential.

Referring now to the output current path, the coils Hi and are purposely so proportioned that the current flowing from source l2 through these coils is maintained substantially constant, irrespective of the variation in impedance of tube '1. However, the space current in tube 7 is allowed to vary, for it is in a path parallel to another path, and as the impedance is varied, there is a variation in the distribution of current through the two parallel paths. In general, this mode of operation of an amplifier is described and claimed in my prior Patent No. 1,594,582, issued August 3, 1926, and entitled Amplifier system. The path paralleling the path in tube l and fed from source I2 includes in this instance also, a space current path. Thus an amplifier tube 23 is provided, having a heated filament l9 fed from source l9, an anode 29, and a control electrode or grid 2i. The space current path then includes source l2, connection l3, coils i l and i5, a coil 22, anode 26, filament i9, and connection ii back to the negative side of source 52. Without further elaboration, it is evident that increase in the space impedance in tube 7, causes a reduction in current flow through elements i8ll in parallel, and a corresponding increase in currer flow in the space of tube 2.3, the sum of the two currents remaining constant due to chokes i l and l5. Conversely, a decrease in the impedance of the path in tube l causes an increase in current flow therein and a corresponding de crease in current flow in the tube The advantages of such eifects have been carefully discussed in my prior patent.

However, as in the prior system, the current difference in the two parallel paths is accentuated, as by aiiecting the input circuit of one amplifier, by the output circuit of the other. For example, coil '22 can be adjustably coupled to coil which coils are respectively associated with the output of tube 23, and the input of tube I.

Now let us assume that an impulse of the desired frequency is picked up by the system. This impulse strongly affects circuit 3; coil 6 carries only a negligible amount of this impulse. The impulse causes the grid l8 to be say, more positive than before. This decreases the impedance oi amplifier l, more current is permitted to pass through it, and less current passes through the electronic device 23. But this decrease in current in tube 223 causes such a change in circuit 3 (due to proper direction of coupling of coils l and 22) that the effect of the impulse on circuit 3 is assisted, and grid 58 becomes still more positive, causing a still greater increase in the diiierence in current flow in the two electronic devices 7 and 23. This building up process continues, and is held in stable position by the degree of coupling of coils and 22.

Upon a reversal of the impulse, grid I8 is made more negative than before; less current flows through device 1 than through device 23; and the increase in current fiow in coil 22 affects circuit 3 so as to reduce the potential of grid l8 still further.

It is thus seen that the current pulsations in coil I? are amplified greatly, and cause correspondingly large variations in potential difierence across its terminals. This potential difference can be made to aiiect a detector 2 5, such as a crystal rectifier or its equivalent. The rectified impulses can be passed through the audio frequency transformer 25, the secondary 26 of which can be connected to several stages of audio fre quency amplifiers, especially in the manner that I shall hereinafter set forth in detail.

It is evident that undesired signal impulse will also have some effect on circuit It is the aim of my invention to cancel this eiiect. For this purpose, coil 6 which is in the antenna circuit and which responds to the undesired impulses, is caused to affect device 23 in the proper way to cause the coil 22 to oppose the undesired impulses in circuit 3. Thus coil 6 can be adjustably coupled to coil 2's in the input circuit of tube 23, and in the right direction for securing the desired result. The degree of coupling is adjusted to provide the desired quantitative effect.

Now let us trace the history of an interfering impulse. Such. an impulse causes coil 4 to respond slightly, and there is a current in coil 4 due to it, which if unchecked, will affect grid I8 and cause the interfering signal to appear in the output circuit of tube 7. Due to the action of coil 6 however, this interfering current is checked or cancelled out; the coil 6 affects grid 2| and in such a direction as to vary the current flow through coil 22 in the right direction to oppose the current in coil 4. Thus the effect of coil 4 on grid 58 is nullified. The very slight controlling variations in current in coil 22 are permitted without disturbing the balance in the two current paths through devices I and 23.

The detector circuit passes the detected impulse through transformer 25 to any conventional form of audio amplifier. The one shown in this case includes two amplifier tubes 28 and 29, and a power tube 3! the last of the series. The input circuit of tube 28 connects to the secondary 26; the filament 3| is supplied with heating current from battery iii, and it as well as grid 32 form the input electrodes. The plate 33 passes the output current through primary 34 of transformer 35. The secondary 36 is bridged by a high resistance 31, and the input of tube 29 connects to a variable portion of this resistance. The output circuit includes primary 38 supplying transformer 3% with impulses, which in turn affects the power tube as input circuit 4! The output circuit of the power tube includes the phones Al or other translating device.

Since the operation of tubes 28, 29 and 3t], is substantially the same as has already been described, they require no further elucidation, except to state that these amplifier tubes are shown in this instance as operating with somewhat higher plate voltages than tubes '7 and 23. Furthermore, the output circuits of tubes 28 and 29 join at point 42, whence they pass through choke coils 33 and M which keep the sum of the currents through the tubes 28 and 29 constant. Therefore these two amplifiers also operate in accordance with the disclosure of my prior patout.

The advantages of the systems as a whole are numerous. But one tuning means need be varied to adjust the system to respond to the desired frequency. The circuit 3 is carefully designed to be critically resonant, whereby substantially all of the desired impulses remain trapped in the circuit. The interfering impulses pass through coil 6, and there initiate forces which cancel out the interfering signals.

I claim:

1. In a selective radio receiving system, a parallel resonant circuit having an inductance and a capacity in parallel, and capable of being tuned to the desired impulses, an amplifier system coupled to said circuit, and an inductance coil in conductive series relation with the circuit, and means whereby the amplifier system is oppositely affected by the undesired impulses in the parallel circuit and in the inductance, comprising an electronic emission device having its input side coupled to the inductance, and its output side coupled to the parallel circuit.

2. In combination, a pair of electronic emission amplifying devices, an output circuit for each of said devices, the two output circuits being in parallel, a source of current supplying said parallel circuits, means for maintaining the current supplied by said source to said parallel circuits substantially constant, means whereby electric signaling impulses serve to vary the impedance of one of said device-s causing the relationship of currents in said two circuits to vary, and means whereby the other device also passes impulses to the input of the said one of said devices and in a direction to accentuate the effect of said signaling impulses, and comprising a coupling between the input of that device which is supplied with signaling impulses and the output of the other device.

3. In an amplifier system, a pair of paths in parallel, a source of current supplying said paths, means for maintaining the current supplied to said paths by said source substantially constant, means responsive to received impulses for varying the impedance of one of said paths, whereby the currents through said paths are varied, and means coupling the paths in such manner that the path whose impedance is varied receives a further impedance variation in the same direction responsive to the variation of the current in the other path.

4. In a receiving system for modulated high frequency signals having an input circuit and two amplifying tubes connected thereto each having input terminals and a plate circuit, the method of confining the overall response of said circuit to desired signals which comprises the steps of maintaining a current in the plate circuit of each of said tubes whose sum is substantially constant, applying substantially all of the potential drop of said desired signals to the input terminals of one of said tubes to cause variations in current in the plate circuit thereof, applying a voltage derived from the variations in plate current of the other tube, caused by the variations in said first mentioned tube, to the input terminals of said first mentioned tube in phase with the original signal, dividing potential drops produced by undesired signals between the input terminals of both of said tubes, and causing the fluctuations of plate current in said second tube caused by said undesired signals to apply an out-of-phase voltage across the input terminals of said first tube.

5. In a system for amplifying waves of a desired frequency and suppressing waves of an undesired frequency, an amplifying vacuum tube and an auxiliary vacuum tube each having a control circuit and an output circuit, means for applying desired and undesired signal voltages to the control circuits of said tubes in different ratios, means for causing variations of current in the output circuit of said amplifying tube to cause variations of current in the output circuit of said auxiliary tube in the opposite sense to variations caused therein by signals in the control circuit thereof, and means for transferring energy from the output circuit of said auxiliary tube to the control circuit of said amplifying tube in such sense as to augment the variations in the output circuit of said amplifying tube caused by desired signals.

PHILIP J. WALSH. 

